R1:310cf00fc3cc

Consolidate feed query code

Summary: Simplify FeedQuery by making it extend from PhabricatorIDPagedPolicyQuery

Test Plan: Looked at feed on home, projects, user profile, and called `feed.query`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2905